There is known a conventional power steering device of a rack assist type such as a below-described patent document 1.
That is, in this ball screw, a plurality balls which are rolling members are circulated through a tube between a pair of ball screw grooves formed on inner and outer circumference portions of a screw shaft and the nut to confront each other. A connection portion between a ball inlet and outlet holes provided in the nut and the ball screw groove (nut side ball screw groove) is processed into a diameter increasing tapered shape. With this, a smooth movement of the balls between the tube and the ball screw groove is ensured.